This invention relates to a frequency hopping spectrum spreading communication device, particularly, to a spectrum spreading GFSK (Gaussian Frequency Shift Keying) communication device capable of maintaining approximately uniform frequency deviation regardless of a carrier frequency.
Generally, a GFSK communication device comprises a VCO (Voltage Controlled Oscillator) to produce a modulated carrier wave as a transmission signal. The VCO is not only used for modulating a carrier wave but also for executing frequency hopping of the carrier wave.
The transmission signal must have frequency deviation restricted within prescribed limits. If a carrier frequency of the carrier wave is fixed, it is easy to restrict the frequency deviation within the prescribed limits. That is, amplitude of modulation voltage supplied to the VCO has only to be restricted within a desirable range in this case.
However, the carrier frequency hops (changes or switches) frequently and takes any one of predetermined hopping frequencies in actuality. In such a case, if the amplitude of the modulating voltage is fixed, the frequency deviation changes according to the change or hopping of the carrier frequency. In other words, the VCO has frequency deviation characteristics depending on the carrier frequency.
A related GFSK communication device comprises a frequency deviation correcting circuit to restrict the frequency deviation within the prescribed limits.
However, the frequency deviation correcting circuit has a complicated structure. For instance, the frequency deviation correcting circuit comprises resistors and switches connected to the resistors respectively. Moreover, it is difficult to control the frequency deviation correcting circuit because there are many combinations of ON/OFF state of the switches.